docs: def disk_vmag(hemi, retinotopy='any', to=None, **kw):
      '''
      disk_vmag(mesh) yields the visual magnification based on the projection of disks on the cortical
        surface into the visual field.
  
      All options accepted by mag_data() are accepted by disk_vmag().
      '''
      mdat = mag_data(hemi, retinotopy=retinotopy, **kw)
      if pimms.is_vector(mdat): return tuple([face_vmag(m, to=to) for m in mdat])
      elif pimms.is_vector(mdat.keys(), 'int'):
          return pimms.lazy_map({k: curry(lambda k: face_vmag(mdat[k], to=to), k)
                                 for k in six.iterkeys(mdat)})
      #TODO: implement the disk_vmag calculation using mdat
      # convert to the appropriate type according to the to param
      raise NotImplementedError()